An episome is a
plasmid (a small, circular,
self-replicating DNA molecule separate from the
bacterial chromosome) capable of integrating into the bacterial
chromosome, a genetic element that can replicate either as a plasmid or as part of the bacterial chromosome.
An important example of an episome is the
F plasmid, which can
undergo reversible
incorporation into the cell's chromosome for
bacterial conjugation. In addition to some plasmids,
temperate virus such as a specific
phage (ex. lambda phage) also qualify as episomes. The
genomes of these
phages replicate sparately in the
cytoplasm during the
lytic cycle, and as an integral part of the host's chromosome during a
lysogenic cycle.